Jamieson, Fausset and Brown on Matthew 5:36

36. Neither shalt thou swear by thy head, because thou canst not make one hair white or black--In the other oaths specified, God's name was profaned quite as really as if His name had been uttered, because it was instantly suggested by the mention of His "throne," His "footstool," His "city." But in swearing by our own head and the like, the objection lies in their being "beyond our control," and therefore profanely assumed to have a stability which they have not.